the type or namespace for hashtable is not found .are you missing an assembly reference?

protected void Page_Load(object sender, EventArgs e)
{
if(!IsPostBack)
{
SqlConnection con = new SqlConnection("Data Source=SureshDasari; Initial Catalog=MySampleDB; Integrated Security=true");
con.Open();
SqlCommand cmd = new SqlCommand("select * from SampleTable", con);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds,"UserDetails");
DataTable dt = ds.Tables["UserDetails"];
RemoveDuplicateRows(dt, "UserName"); // Here UserName is Column name of table
gvDetails.DataSource = ds;
gvDetails.DataBind();
}
}
// This method is used to delete duplicate rows of table
public DataTable RemoveDuplicateRows(DataTable dTable, string colName)
{
    Hashtable hTable = new Hashtable();
    ArrayList duplicateList = new ArrayList();
    foreach (DataRow dtRow in dTable.Rows)
    {
        if (hTable.Contains(dtRow[colName]))
            duplicateList.Add(dtRow);
        else
            hTable.Add(dtRow[colName], string.Empty);
    }
    foreach (DataRow dtRow in duplicateList)
        dTable.Rows.Remove(dtRow);
    return dTable;
}

using System;
using System.Configuration;

using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using MySql.Data.MySqlClient;
using System.Text;
using System.Net;
using System.Net.Mail;
using System.Data;